(a) How does primary succession proceed in a lake to develop into a forest? Explain.

(b) Why do ecological successions take several thousands of years? Explain giving reasons.

OR

(a) Explain with the help of an example the upright pyramid of biomass. How is the pyramid of biomass in a sea different from the one explained?

(b) List two limitations of ecological pyramids.

a) During the primary succession the pioneer species are the small phytoplanktons which are later replaced by free floating angiosperms, then with time they were replaced by rooted hydrophytes, sedges, grasses and finally the trees.

Ultimately the climax species were trees forming a forest.

b) Ecological successions take several thousands of years due to the long time it takes for progression from one species to another in the sere.

e.g. At the origin of Earth there was lava all over which took long time to cool the took time to form hard rock then ttok long time for weathering i.e. conversion of rock to smaller rocks then to stones then to gravel and then finally formation of soil.

OR

a) The pyramid of biomass of terrestrial ecosystem is upright i.e. broad at bottom and tip at the apex. It is so due to large number of producers and lesser number of consumers.

The pyramid of biomass of aquatic ecosystem is inverted as biomass of planktons is extremely low in comparison to biomass of fishes.

b) The two limitations are:-

i) It takes into account only simple food chains which do not exist in nature.

ii) It does not take into account the species which belong to more than one trophic level.